The use of adaptive or smart antennas is considered as one of the key features for increasing coverage and capacity of a wireless system such as WCDMA. When beamforming is applied in the base station several narrow beams, compared to the sector/cell beam, may be created to maintain coverage in the cell. FIG. 1A illustrates a sector cell antenna beam. Although a sector antenna is useful to communicate broadcast and/or control information to all mobiles in the sector cell, an adaptive antenna may be used to transmit and receive in narrow beams covering just a part of the sector cell. FIG. 1B shows an example of a narrow antenna beam. FIG. 2 illustrates an example of a cellular network with a base station transmitting a sector beam, a base station transmitting one of the possible beams in a multi-beam system, and a base station transmitting a steerable beam. Some benefits of adaptive antennas are shown in FIG. 3, where a narrow beam of the adaptive antenna may be directed to an intended mobile and therefore spreads less interference in the download or downlink direction. The narrow beam also suppresses spatial interference from adjacent cell interferers in the uplink direction. Both factors increase the signal-to-interference gain, and therefore increase the overall system performance. Currently there exist no commercial installations of adaptive antennas in the field of WCDMA systems.
Another fundamental feature in a cellular network is handover (HO). Handover is a main function that is used to support mobility in the network. When a user is moving in the cellular network, it has to change serving cell when the signal from the current cell is too weak to support the current radio link. Also, in e.g. WCDMA there is a soft handover (SHO) functionality. This means that there may exist several radio links between the user terminal and the network. This has the effect that the combination of several radio links together may provide sufficient quality of the combined radio link set.
When narrow beams are created by the smart antenna system, the user equipment (UE) needs to make a handover between the different beams in order to maintain the call. In order to make these handovers, a reference (or pilot) signal measurement is used to decide upon the quality of a specific base station antenna beam.
However, when adaptive antennas and handover are combined in wireless networks such as WCDMA certain problem arises. In many systems, e.g. WCDMA, a common pilot signal is used as a reference for the handover measurements. This reference should be transmitted in the whole cell (defining the cell coverage), as specified by 3GPP standards. Hence the reference is common for an entire cell and may thus constitute a poor reference for the quality in specific beams when beamforming is applied. Furthermore, when performing a handover, the narrow beams of the adjacent cells are not known to the mobile, hence the handover needs to be carried out on sector covering wide beams.
In reference [1], downlink quality measurements associated with a broadcast signal transmission from neighboring base stations are detected by the mobile and reported to the radio network. First, a target base station is determined based on the measurements, and a radio link is established between the target base station and the mobile using the cell-wide broadcast signal. Subsequently, a desired antenna beam is determined using uplink measurements, and the radio link is then re-configured to the desired antenna beam. This means that after establishing a radio link, the link needs to be re-configured onto a narrow beam. When a new handover should be performed, each radio link must first be re-configured to a sector-wide beam again. This process will not only consume resources in the network for signaling, but will also lower the capacity since some radio links are transmitted over a wide sector beam instead of a high gain narrow beam.
There is thus a general need to improve the handover function in wireless systems when adaptive antennas and/or beamforming is applied.